Bob Thrun wrote:Polyester rope is slightly heavier and slightly less strong than nylon. Nylon loses about 10% of its strength when wet. Polyester does not lose strength when wet. Polyester has less stretch than nylon.

Why are poly ropes not the cavers standard? Especially in this country where elegant rigging is the exception?

It's a matter of personal preference. I have used PMI Isostatic a fair amount, and I just feel safer with nylon. My concern is shock loading - admittedly rare - but when it does happen, I prefer the stretch of PMI Max Wear or Flex over PMI's Isostatic. My concern is regarding the anchors failing, not the rope breaking, BTW.

Makes sense. I guess my appreciation of this rope comes from the sort of use I put it to. We're mostly doing exploration and abusing ropes terribly. When you're rigging around chunks of rock and throwing the rope down a tight jaggedy hole with 47 rub points having zero stretch seems very attractive.

There's a reason why nylon static rope has 5-6% stretch. Personally, I would reserve low stretch polyester ropes for situations which specifically call for their use, and not as an everyday caving rope. The reason why it's not the standard is that when using it you essentially want to never have a fall factor of any amount, and that's a slim margin for error, both for the user and the anchor.

LukeM wrote:There's a reason why nylon static rope has 5-6% stretch. Personally, I would reserve low stretch polyester ropes for situations which specifically call for their use, and not as an everyday caving rope. The reason why it's not the standard is that when using it you essentially want to never have a fall factor of any amount, and that's a slim margin for error, both for the user and the anchor.

I've always assumed that the stretch of static is a result of the materials used, not a design characteristic. My wife and I have been sitting here trying to imagine scenarios in which a caving rope would experience a shock load. They all involve catastrophe, bad anchors, or bad judgment. Would the stretch of nylon help in the case of, for example, a rebelay blowout? Probably so. So I would agree with you that super static ropes are not for every situation. On the other hand, I believe that they are suitable and even preferable for what amount to everyday caving situations for many of us.

I would like to know what demonstrable differences there are when these ropes are shockloaded. My only means of testing would be extremely crude, but I have some scraps available and may make comparative drop tests on tenuous anchors.

GroundquestMSA wrote: I've always assumed that the stretch of static is a result of the materials used, not a design characteristic.

Au contraire; consider nylon climbing and caving ropes, the former having high stretch and the latter nearly static. The only significant difference is the construction - climbing ropes typically have a twisted core, while caving ropes have either parallel- or braided-strand cores. Caving ropes may also have a heavier, tighter sheath. Bottom line, however, is that the climbing rope will have a static elongation of maybe 8-10%, and 30+% dynamic stretch, while the caving rope has 1-3% static stretch and next to no dynamic. Both nylon.

Changing materials, however, can have significant impact as well - as per this whole discussion about nylon/Spectra/Dyneema/polyester, etc. Steel cable is even more static, albeit with considerably poor handling. (FWIW, I used to have a polyester climbing harness made by Chouinard. One of the benefits was the that polyester doesn't lose strength when wet, at least as much as does nylon. Nice harness, by the way.)

GroundquestMSA wrote: My wife and I have been sitting here trying to imagine scenarios in which a caving rope would experience a shock load. They all involve catastrophe, bad anchors, or bad judgment. Would the stretch of nylon help in the case of, for example, a rebelay blowout? Probably so. So I would agree with you that super static ropes are not for every situation. On the other hand, I believe that they are suitable and even preferable for what amount to everyday caving situations for many of us.

I can envision (and have experienced) numerous situations where you can shock load a caving rope. Blowing an anchor is an obvious example, which you mentioned. Not so likely in well-traveled, strongly bolted caves, but light, remote push work often involves compromise anchors. Another common scenario are lower-angle and/or tight pitches, where one often tends to climb the rock as much as climbing the rope; it is pretty easy to climb up a few feet and realize that you have slack rope riding up with you. Neither of these is likely to result in a high fall factor, but still a shock load. Climbing above an anchor is another common problem, one that should always be avoided, but isn't always avoidable - and one that can end up with a fall factor greater than one.

All that said, I think that in general, lower stretch usually means less abrasion and more efficient climbing - and so is usually preferable. Additionally, the some of energy generated in 'shock' loading a rope is going to be mitigated by your harness shifting, your body bouncing around, etc., and is a good argument for using dynamic rope for cowtails.

GroundquestMSA wrote: I've always assumed that the stretch of static is a result of the materials used, not a design characteristic.

Au contraire; consider nylon climbing and caving ropes, the former having high stretch and the latter nearly static. The only significant difference is the construction - climbing ropes typically have a twisted core, while caving ropes have either parallel- or braided-strand cores. Caving ropes may also have a heavier, tighter sheath. Bottom line, however, is that the climbing rope will have a static elongation of maybe 8-10%, and 30+% dynamic stretch, while the caving rope has 1-3% static stretch and next to no dynamic. Both nylon.

What I really meant is that I assumed, since all the super-static ropes seem to be poly or something other than nylon, that the slight stretch of nylon caving rope was unavoidable, not an intentional safety design. This assumption too may be wrong.